Field of the Invention
The present invention relates generally to a system for lowering noise level in a vehicular cabin. Particularly, the invention relates to a system for canceling noise created by acoustic vibration generated in synchronism with an engine revolution by generating acoustic vibration suppressing or at least reducing amplitude of the noise created by the acoustic vibration.
A system for canceling noise created by acoustic vibration, which will be hereafter referred to as "noise vibration", by generating acoustic vibration which will be hereafter referred to as "noise canceling vibration" adapted for at least partly canceling noise created by acoustic vibration, has been disclosed in Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 62-127052. In this prior proposal, a rectangular wave signal is generated in relation to a spark ignition signal in the form of a pulse signal, because the spark ignition signal has a period corresponding to the noise vibration. In order to maintain the duty cycle of the rectangular pulse at 50%, a pulse width of the rectangular wave signal is set at a half of an interval of leading edges of the spark ignition pulses in the immediately preceding cycle.
The rectangular wave signal thus generated is subject to phase treatment and then converted into a sine wave signal. The sine wave signal is amplified by an amplifier. A control signal for performing amplification for the sine wave signal is an analog signal derived through a digital-to-analog conversion.
In such prior proposed system, a microprocessor is used for processing the spark ignition pulses for deriving the pulse width in order to maintain the duty cycle of the rectangular wave signal substantially at 50%. Furthermore, a digital-to-analog converter for forming the analog control signal is required. Both of the microprocessor and the digital-to-analog converter are relatively expensive resulting in a high cost of the overall system. On the other hand, when amplification of the sine wave signal is performed by an analog amplifier, fluctuation of linearity and phase characteristics of amplification degree can become unacceptable.